This dissertation is divided into three chapters. The first chapter deals with an unprecedented cyclization that occurs during the reaction of norbornadiene-palladium dichloride and 1-iodomercurio-2-butyne. This reaction yields two novel [sigma]-vinylpalladium compounds. The work in chapter one focuses on obtaining further support for the two proposed structures. The second chapter deals with palladium-catalyzed arylannulation of conjugated dienes. Appropriately substituted aryl iodides and 1,3-dienes undergo a one-step, palladium(0)-catalyzed reaction to furnish polycycles. During arylannulation two carbon-carbon bonds are formed under relatively mild conditions. Chapter three is an extension of the work in the second chapter. Palladium-catalyzed arylannulation of non-conjugated dienes, alkynes, and simple olefins have been examined, and polycycles are formed under relatively mild conditions.